Potential of concentration-response data to broaden regulatory application of in vitro test guidelines

Abstract

International chemical regulatory activities are moving towards new approach methodology and away from traditional animal-based models, shifting and expanding from one single in vivo assay towards combined use of different in vitro assays within integrated approaches for testing and assessment and defined approaches to serve hazard identification, classification and selection of points of departure for risk assessment. Whilst many in vitro test guidelines were developed against specific hazard cut-off values, quantitative information is needed in data interpretation procedures for potency assessment purposes or to define points of departure so that assays can fulfill evolving regulatory needs. Utilizing four examples from skin sensitization, phototoxicity, endocrine activity, and nongenotoxic carcinogenicity, we illustrate why a shift in data generation and data interpretation procedures is needed to facilitate the full exploitation of the data that is generated using these assays. This requires the development of a practical approach that uses or expands upon existing guidance. Experience gained with such an approach can then provide a basis for an overarching strategy in test guideline development that should better facilitate combinations of in vitro test guidelines for specific endpoints that will be more transparent, robust, and adaptable for specific regulatory purposes.

Development of a reference and proficiency chemical list for human steatosis endpoints in vitro

“…Additionally, it is being recognised that concentration-response data, that are often already recorded in non-animal alternative methods, but not yet utilised for regulatory purposes are key to leverage in vitro test methods beyond prioritisation and regulatory hazard identification ( 66 ). It was intended that the tentative proposed chemicals for human in vitro steatosis test method optimisation, proficiency, and (pre-)validation testing provided in this study cover chemicals with a range of activity (negative, low, moderate, high induction potential) towards steatosis.…”

“…Chemical potency can be used in the context of an AOP to inform as to whether an adverse effect at the molecular level leads to higher cellular and tissue effects, i.e., if it surpasses the “tipping point” leading from one key event to the next, or if physiological adaptation and compensatory mechanisms can prevent adversity at higher levels ( 62 – 66 ). In the context of an IATA, such potency information could be used to inform if or which higher-tier tests are needed to confidently conclude on chemical hazard characterization.…”

“…In the context of an IATA, such potency information could be used to inform if or which higher-tier tests are needed to confidently conclude on chemical hazard characterization. For regulatory purposes, chemical potency information can be a first indicator to inform upon the derivation of a toxicological reference dose (RfD), and regulatory exposure limits such as the acceptable daily intake (ADI) ( 66 ).…”

Candidate Proficiency Test Chemicals to Address Industrial Chemical Applicability Domains for in vitro Human Cytochrome P450 Enzyme Induction

“…In the latter case, estimations were considered to be of lower reliability as compared to cases where cell-based enzymatic activity data was available, and insufficient data were considered to be available to estimate the effect of a chemical in the test method if overall the available good quality data was considered to be contradictory. In the design of validation experiments intended for applications beyond classification and prioritisation purposes, it is good practice to include proficiency chemicals that are expected to produce a potency range from negative to low, moderate, and strong effects, and to generate concentration-response information, as this is of greater utility for IATA approaches ( Jacobs, Ezendam et al, 2022 ). Thus, for chemicals for which cell-based enzymatic activity data showed them to be an inducer, the magnitude of any observed CYP enzyme induction was categorized as low (≤3 fold), moderate (>3 to 4.5 fold), or strong (>4.5 fold), and this information was used in the chemical selection considerations.…”

A modular strategy for the testing and assessment of non-genotoxic carcinogens

“…Furthermore, the integration of results that visualise a biological continuum can reduce uncertainty regarding the Point of Departure' (PoD) as one moves from MIE to subsequent KE testing. It is also recognised that not all the in vitro test methods within the IATA are currently optimised such that they can provide concentration responses that can be utilised as potency scores (Jacobs et al 2022a, b). Instead, they are optimised for conventional assessments, which require binary test outcomes that have been obtained from outcomes of the biological continuum (similar to, for instance, the number of revertants in the Ames assay, or stimulation indices in skin sensitisation assays).…”